We will continue our morphological, physiological, and immunochemical studies of the structure and function of the gastrointestinal tract and the pathogenesis of its diseases. How phospholipids influence fat absorption in the human small intestine will be examined in jejunal segments isolated by the proximal occluding balloon technique. How rates of absorption and aqueous solubility of fatty acids determine their routes of transport from the small intestine will be determined in rats and in man. The influence of luminal factors on synthesis and release of intestinal lipid particles will be studied in segments of rat small intestine in vitro. The synthesis of apoproteins by human absorptive cells will be examined by immunochemistry and immunomorphology. How luminal pH, Ca ions, Mg ions, and short-chain fatty acids affect water and solute absorption will be investigated in isolated, infused segments of rat intestine. The role of extracellular leaks through the intestinal mucosa in the pathogenesis of secretory diarrheas will be studied in animal models and in naturally occurring human diarrhea. The effect of chronic laxation on the structure and function of the mucosa, smooth muscle, and myenteric plexus of the colon will be examined in patients, and in an animal model where the colon is chronically exposed to laxatives. Abnormalities of structure and function of the mucosa, smooth muscle and myenteric plexus will be defined in patients who have diseases associated with impaired intestinal propulsion such as idiopathic intestinal pseudoobstruction.